1. Field of the Invention
The invention provides valves of the type employing a differential pressure regulator and a characterized valve with a mechanism to enhance control of fluid flow independently of system pressure. The valves of the invention are especially suitable for HVAC applications, and provide operation with improved accuracy over a greater flow range with a reduced risk of hunting.
2. Description of Prior Art Related to the Invention
In HVAC plants, water distribution can be accomplished at constant or variable flow. Each type of distribution system has advantages and disadvantages. Today, variable flow systems using electronic 2-way control valves have become generally accepted as the industry standard due to their benefits, mainly reduced pumping cost achieved as a result of reducing pump head and flow.
However, a significant disadvantage with many systems in place today is that balancing the system is a time consuming and costly effort. The balancing procedure dictates the quality of the system and requires highly skilled technicians and tools. According to a typical system flow design, each control valve requires a balancing valve to adjust the hydronic circuit with all control valves must be in their open position. Then, as soon as the system is running, depending on different cooling or heating load requirements in the building, valves begin closing and opening. Each opening or closing dynamically alters system pressure, and balancing these systems becomes time consuming and can only be approximate.
If terminals are added to a typical system, the whole system requires rebalancing because some existing terminals must be throttled back. This is especially a problem where floors are periodically remodeled and the uses are varied. Wherever the uses change, balancing of the whole system is required. Moreover, a building is typically running under design conditions only a small of the time. The rest of the time, the hydronic system needs to provide an average load of only about 50%. This can mean that flow is reduced to only about 20%, and differential pressures across control valves increase. Since the CV-rating of the valve is typically sized for design conditions, the valve authority decreases and the modulating valve is downgraded to one acting open or closed only. This makes hunting expected.
Control circuits are interactive. Therefore, when one control valve closes in a system of the type described above, the differential pressures on other circuits increases and the associated control valves must close to compensate. So when one or more loops become unstable, control problems can spread to other control valves.
In typical current cooling systems, if flow is higher than required, the temperature differential of the fluid (ΔT) will decrease and result in a cooling plant with lower return temperatures to the chiller and reduce the efficiency. If one chiller cannot run at peak efficiency, it is more likely that the next chiller in a series will be forced to start sooner than required causing additional electricity and maintenance costs. The opposite happens in a condensing boiler where a higher return temperature can avoid the condensing process when the dew point of the exhaust gases cannot be achieved. The same phenomenon can happen in coils. In a heating coil for instance, overflow will result in a lower ΔT and decrease the coil's performance which can result in discomfort due too a low room temperature.
Significant developments in HVAC valves have been made in the recent past with the provision of characterized valves, in general, and particularly of the pressure independent variety. Because of these improvements, many of the disadvantages of variable flow systems are largely eliminated for most HVAC-applications. The valves now available for HVAC applications include characterized openings where the degree of opening movement is proportional to flow rate. U.S. Pat. No. 6,039,304, to Carlson, et al., describes a ball valve with modified characteristics. The valve includes a disk for characterizing flow to permit a proportional of opening of the valve to correspond to a predetermined flow rate. These valves can provide essentially “equal percent” characteristics, as well as other flow characteristics, and are commercially available under the identifier of CCV. They employ a disk having a shaped opening with one side shaped to conform to the shape of the exterior of the ball or plug. The disk fits inside the port at the seat area, and is secured by a ring. The ability of a valve to provide a flow rate proportional to the movement of a valve actuator is of great advantage when manufacturing and installing both valves and the actuators.
It is also important for HVAC and other applications that control valves have the ability to maintain a constant flow rate despite pressure fluctuations in the system. Valves having this capability are now available. In U.S. Pat. No. 6,827,100, to Carlson, there is described a pressure independent control valve, which enables an HVAC operator to set flow rates for any of a plurality of zones and have the selected rates remain constant independent of variations in pressure due to variations in heat transfer demand in the several zones. These valves are commercially available under the identifier of PICCV. Pressure independent control valves of the type described in U.S. Pat. No. 6,827,100 work well over the greater part of their rated nominal flow; however, at certain pressures and flow rates, there is sometimes, especially in closed systems, a risk of oscillation.
At part-load, for instance, when a PICCV with a nominal flow of 10 GPM operates at 3 GPM, a flow of 3 GPM is maintained, and these valves provide good control with no adverse hunting. However, at higher flow rates in closed systems, oscillation can be a problem. While the problems can be addressed by providing a wide range of valve sizes and selecting valves always having an effective range, so that there would always be one that worked at the problem flow rates and pressures, it would be desirable to control the problem more directly and enable a smaller range of valve sizes to accomplish the same tasks.
There is a present need for means having the capability to simply and effectively modifying pressure independent control valves to enable to enhance control of fluid flow independently of system pressure with improved accuracy over a greater flow range with a reduced risk of hunting.